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  <div class="titlepage"><div><div><h1 style="clear: both">Limitations</h1></div></div></div>
  <p>C++ has excellent two-way compatibility with the C ABI, but there are some
limitations you must observe to write C++ code which C code can call without
marshalling at the ABI boundary:</p>

<ol>
<li><p>A C++ function may not throw exceptions if it is safe to call from C, and
so should always be marked <code>noexcept</code>.</p></li>

<li><p>A C++ function should be annotated with <code>extern &quot;C&quot;</code> to prevent its symbol
being mangled, and thus give it the C rather than C++ ABI.</p></li>

<li><p>You cannot use overloading in your <code>extern &quot;C&quot;</code> functions.</p></li>

<li><p>You may only use types in your C++ function declaration for which these traits are both true:</p>

<ul>
<li><a href="http://en.cppreference.com/w/cpp/types/is_standard_layout"><code>std::is_standard_layout_v&lt;T&gt;</code></a></li>
<li><a href="http://en.cppreference.com/w/cpp/types/is_trivially_copyable"><code>std::is_trivially_copyable_v&lt;T&gt;</code></a></li>
</ul>

<p>(Note that <code>std::is_trivially_copyable_v&lt;T&gt;</code> requires trivial destruction,
but NOT trivial construction. This means that C++ can do non-trivial construction
of otherwise trivial types)</p></li>
</ol>

<hr />

<p>The above is what the standard officially requires for <em>well defined</em> C and C++ interop.
However, all of the three major compilers MSVC, GCC and clang are considerably more relaxed.
In those three major compilers, &ldquo;almost-standard-layout&rdquo; C++ types work fine in C.</p>

<p>&ldquo;Almost-standard-layout&rdquo; C++ types have these requirements:</p>

<ol>
<li>No virtual functions or virtual base classes i.e.
<a href="http://en.cppreference.com/w/cpp/types/is_polymorphic"><code>std::is_polymorphic_v&lt;T&gt;</code></a>
must be false. This is because the vptrs offset the proper front of the data layout
in an unknowable way to C.</li>
<li>Non-static data members of reference type appear to C as pointers. You
must never supply from C to C++ a non-null pointer which is seen as a reference in C++.</li>
<li>C++ inheritance is seen in C data layout as if the most derived class has nested
variables of the inherited types at the top, in order of inheritance.</li>
<li>Types with non-trivial destructors work fine so long as at least move construction
and assignment is the same as
copying bits like <code>memcpy()</code>. You just need to make sure instances of the type return
to C++, and don&rsquo;t get orphaned in C. This was referred to in previous pages in this
section as &ldquo;move relocating&rdquo; types.</li>
</ol>

<p>Experimental Outcome&rsquo;s support for being used from C does not meet the current strict
requirements, and thus relies on the (very common) implementation defined behaviour just
described (it is hoped that future C++ standards can relax the requirements to those
just described).</p>

<p>Specifically, proposed <code>status_code</code> is an almost-standard-layout type,
and thus while it can&rsquo;t be returned from <code>extern &quot;C&quot;</code> functions as the compiler
will complain, it is perfectly safe to return from C++ functions to C code on the
three major compilers, as it is an &ldquo;almost-standard-layout&rdquo; C++ type if <code>T</code> is
an &ldquo;almost-standard-layout&rdquo; C++ type.</p>


        </div><p><small>Last revised: February 05, 2019 at 17:14:18 UTC</small></p>
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